I will just give my personal opinions on my favorite mods/custom stories:

1. The Streets of London
2. Enigma
3. Penumbra: Necrologue
4. Penumbra: Twilight of the Archaic
5. In Lucy's Eyes

These may not be unique, but they are very well made. Don't worry if Streets of London crashes, if you just go to continue it will work, at least it did for me. Though I do have a great computer. So be aware of that.